Sandwich composite structures were introduced to the aviation industry because of their light weight, which makes it possible to reduce the weight of structures compared with structures made using metal materials. In addition, such composite structures present the advantage of avoiding any problem of corrosion.
The introduction in particular of composites constituted by preimpregnated fabric based on carbon fibers and thermosetting matrices implemented by molding in an autoclave or by means of a vacuum bag in a stove, has given satisfaction in particular in the field of aviation (fuselage, tail plane, tail fin, cowlings, blades, propellers, transmissions, suspension, . . . ). The mechanical characteristics (stiffness, strength) obtained with such composite materials when subjected both to static loading and to fatigue loading have been found to be at least equivalent and often better than the characteristics obtained with metal materials (light alloys, titanium, steel, . . . ) with a weight saving that can be as great as 30% in some cases.
In general, preimpregnated fabric is constituted by a non-polymerized thermosetting matrix impregnating reinforcement at some determined impregnating weight fraction (generally 25% to 50%). The main preimpregnated fabrics commonly in use in the aviation industry are constituted by textile reinforcement based on glass, carbon, or aramid fibers, together with epoxy, phenolic, or bismaleimide resins.
Those preimpregnated fabrics are stored at temperatures close to −18° C. so as to avoid initiating the exothermic polymerization reaction of the impregnation matrix. They are cut to shape and laid manually or automatically at ambient temperature.
The operation of draping the composite structure consists in juxtaposing various plies of preimpregnated fabric in a sequence and at orientations that are well determined. The textile reinforcement of the various plies of preimpregnated fabric is preferably oriented in different directions (0°, ±45°, 90°, . . . ). These directions are defined essentially as a function of the nature of the mechanical loads to which the composite structure is subjected (traction, bending, shear, or other), and also as a function of the directions of said loads.
Once the various plies of preimpregnated fabric have been draped, they are subjected to molding in an autoclave or molding with a vacuum bag in a stove in order to obtain the final composite part.
The autoclave method is used for fabricating high quality structural parts having a high volume fraction of fibers, greater that or equal to 55%, and low porosity, less than 3%. The autoclave is a vessel under pressure in which the conditions for curing, polymerizing, or cross-linking the impregnation thermosetting matrix are optimized by controlling vacuum, pressure, rate of temperature rise, and curing temperature. Molding preimpregnated fabric in an autoclave is used in particular for molding composite parts that are thick and complex in shape. The vacuum bag method is more particularly suitable for composite parts that are thin and of large dimensions, and that do not require a high fiber volume fraction. Under such circumstances, the composite part is polymerized in a stove, in a vacuum, solely after a relatively short curing cycle.
A sandwich composite structure is made up of fine skins provided with a plurality of preimpregnated fabric plies positioned on either side of a core, the core being made of a foam-based material or being shaped as a honeycomb, for example. Bending, traction, and compression stresses are withstood by the skins, while shear stresses are withstood by the core. That type of architecture serves in particular to reduce significantly the weight of the final part while conserving a high level of mechanical performance. That type of sandwich construction presents stiffness and bending strength that are particularly great.
Nevertheless, one of the more common modes of mechanical failure inherent to that structural concept lies in the skins separating from the foam or honeycomb core. Bonding between the skins and the core is a key factor in ensuring structural mechanical integrity for the part. In the process of dimensioning a sandwich composite structure, a specific mechanical peeling test is performed in order to characterize the quality of the interface and the bonding between the skins and the core.
Amongst the preimpregnated fabrics that are used in constructing sandwich type composite structures, two distinct categories can be distinguished.
The prior art teaches firstly that a preimpregnated fabric has a single impregnation matrix having physico-chemical and rheological properties that are generally isotropic disposed on the top and bottom faces of reinforcement, the matrix not being self-adhesive. In the particular circumstance of sandwich composite structures, it is necessary to add a film of adhesive between each face of the core and each ply of preimpregnated fabric in contact with said faces, in order to obtain an interface of good quality. The adhesive film is generally expensive, and requires a prior processing step, thereby lengthening fabrication time. Furthermore, because of its large weight per unit area, which for epoxy type adhesives can lie in the range 300 grams per square meter (g/m2) to 450 g/m2, such an additional film of adhesive can sometimes increase the weight of the final sandwich composite structure significantly (up to as much as 30%). Those drawbacks of increasing weight and cost are particularly pronounced with skins of small thickness, i.e. possessing fewer than about ten preimpregnated fabric plies.
To remedy that type of problem preimpregnated fabrics having self-adhesive matrices on their top and bottom faces have been formulated and are commonly used for producing sandwich composite parts.
Nevertheless, they remain rather expensive because of the specific ingredients introduced in their formulation in order to obtain self-adhesive properties. Those self-adhesive preimpregnated fabrics are used above all for skins of small thickness, generally constituted by fewer than about ten plies of preimpregnated fabric.
Self-adhesion results from adding one or more plasticizers which combine with the various monomers and setting agents of the non-cross-linked thermosetting prepolymer of the matrix. They enable the matrix to have viscosity and rheological properties that encourage bonding between the preimpregnated fabrics and the core, when the polymerization reaction begins.
Those preimpregnated fabrics constituted by an self-adhesive matrix including such plasticizers thus enable sandwich composite structures to be obtained without an adhesive film at the interface between the skins and the core of foam or honeycomb, thereby reducing the weight thereof.
Unfortunately, by reducing in particular the cross-linking density in the three-dimensional lattice, those plasticizers lower the glass transition temperature and also lower the mechanical performance of the composite structure at high temperature. The glass transition temperature of a composite constitutes a characteristic that is essential for dimensioning it relative to its environment. When the external temperature becomes higher than the glass transition temperature of the cross-linked matrix, then the stiffness of the composite collapses quickly.